The invention relates to the field of optical communications and the routing of different wavelength signals. In particular, the invention relates to metal-free reflective diffraction gratings that are used in optical communications for dispersing light by wavelength.
Optical communication systems include, among others, elements such as multiplexers, devices that route different wavelength signals from individual pathways into a common pathway, and demultiplexers, devices that route the different wavelength signals from a common pathway back into individual pathways. Often, the only difference between these two devices is the directions of the light traveling through them.
The multiplexer/demultiplexer designs that have gained widest acceptance are based on spectrographs containing either phased arrays or diffraction gratings. Within these two types of devices, the two mechanisms that arc used for routing the optical signals between the common and the individual pathways are dispersion and focusing. Dispersion angularly distinguishes between the different wavelength signals. Focusing converts the angularly distinguished signals into spatially distinguished signals. While phased arrays work well when different wavelengths are conveyed among a small number of optical channels (pathways or waveguides), they become unwieldy when a large number of channels are involved. Diffraction gratings are better suited for systems having a large number of channels. It is the ability of diffraction gratings to map wavelengths into a propagation angle that makes them suitable for use in add-drop filters, multiplexers and demultiplexers, and other wavelength configurable devices.
Typical reflective diffraction gratings involve the use of metal-coated surfaces to provide the reflectivity. Metals commonly used in such gratings are, among others, gold, silver, aluminum and nickel. However, using a metal to provide the reflective properties of the grating results in a reduction in the maximum attainable diffraction efficiency due to the absorption of light by the metallic surface. Typical maximum attainable efficiencies are limited to 90-95% of the theoretical maximum and depend on the specific metal (e.g., Ag, Au, etc.) that is used and the wavelength of the incident light. Another problem with metal-coated diffraction gratings is that the absorbed light is converted into heat which can create problems, including catastrophic failure of the device, when the grating is used in applications that require high optical power. Lastly, in addition to the heat problem, the manufacturing of a grating with a metalized surface is a multi-step process that involves additional equipment, materials, time, and cost.
In addition to the foregoing problems with metalized gratings, the metalization process itself is complicated, has many problems and is expensive to use. For example, consider FIG. 1 representing an idealized grating of the type known in the art and the case where a material 1 of refractive index na is air and a material 2 of refractive index nm is some metal, such as gold or aluminum. The typical fabrication of such a grating involves creating a master grating from a suitable selected material such as glass, depositing a release layer on this master, applying the metallic coating to the release coated master, and bonding the metal to a substrate using an adhesive. Once the adhesive is cured, assuring adhesion of the grating form to the substrate, the master can be released and the metal remaining on the substrate acts as the grating surface (material 2) where light is incident from air (material 1). In a further example, consider the case where material 1 is not air but rather some dielectric such as glass or a polymer. In this case an adhesive layer must first be deposited on the grating surface to ensure that the metal deposited in a subsequent step remains attached to the surface of material 1.
In either of the foregoing cases, the required multi-step metalization process is made additionally complicated by the fact that the surface to be coated is not smooth, but can have very fine and possibly deep structures associated with the grating profile. The deposition processes must ensure that the coating is properly distributed over all surfaces regardless of the grating profile. This task that is very difficult for gratings of high aspect ratio, defined as the ratio of the structure depth to the structure width. Additionally, the adhesion of the metal is problematic when the complete grating undergoes rigorous environmental testing; for example, temperature cycling and/or aging at 85% relative humidity at 85xc2x0 C.
M. S. D. Smith et al, xe2x80x9cDiffraction gratings utilizing total internal reflection facets in Littrow configuration,xe2x80x9d IEEE Photon. Tech. Lett. Vol. 11, (1999), pages 84-86, proposed that for an immersed grating with one material being air and a second being a dielectric with index greater than that of air, if the grating profile is shaped such that incident light reflecting off any facet of the surface is reflected via total internal reflection (xe2x80x9cTIRxe2x80x9d), then high-efficiency diffraction can be attained without a metallic coating. However, the manufacturing process for such a grating requires precise control of the grating profile being made, and the process must be drastically changed each time one fabricates a grating with different functionalities and/or profile. In addition, it limits the allowable angles of incidence that can be used for the grating due to grating surface designs that can be actually, in practice, be fabricated. As a result, such grating has not become commercially available or widely used.
Consequently, in view of the foregoing problems it would be highly desirable to have a reflective diffraction grating that does not have a metalized reflective surface and does not require TIR from every facet of the grating surface.
The invention is directed to a metal-free reflective immersed diffraction grating for optical communications that is made of at least a first material 1 of refractive index n1 and a second material 2 of refractive index n2, wherein light from material 1 is incident on the grating; and the conditions of the following Expression (I)-(III) are met:
n1 greater than n2,
n1 greater than xcex/2L  greater than n2 for single diffracted order at Littrow, and
n2/n1 less than Sin xcex8j less than 1;
wherein xcex is the wavelength of the light incident on the grating, xcex8j represents any and all propagation angles of incident and diffracted light and L is the grating period, and wherein said grating profile is located at the interface of material 1 and material 2. Material 1 can be a glass, a polymeric or copolymeric material, a crystalline structured or amorphous optical material, or a semiconductor such as silicon. Material 2 can be a glass, a polymeric or copolymeric material, a crystalline structured or amorphous optical material, a semiconductor such as silicon or a gas or vapor including air selected such that Expressions (I)-(III) are met.
In another embodiment of the invention, the grating profile is formed of additional materials 3 and 4, and the grating so formed is placed between materials 1 and 2 described above. Further, materials 3 and 4 can have any refractive index value provided they are not identical.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are in tended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.